Method for synthesizing n-type diamond having low resistance

ABSTRACT

In synthesizing a diamond by a vapor-phase growth method, a sputtering method, or a high-pressure and high-temperature synthesis method, N, P or As as an n-type dopant, and H as a p-type dopant are simultaneously doped in a crystal to form a donor-acceptor pair in the crystal, to thereby synthesize a transparent n-type diamond having low resistance.

FIELD OF THE INVENTION

This invention relates to a method for synthesizing an artificialdiamond, and more specifically, to a method for synthesizing atransparent n-type diamond having low resistance in manufacturing adiamond by a vapor-phase growth method, a sputtering method, ahigh-temperature and high-pressure synthesis method, or the like.

TECHNICAL BACKGROUND

As a method for synthesizing a diamond, a method for artificiallysynthesizing a diamond from a graphite carbon by using a catalyst underhigh-temperature and high-pressure (50 kbr, 1500 k or more), and amethod for synthesizing a diamond thin film on a substrate by a CVDmethod from a mixed gas of a hydrocarbon and a hydrogen under lowpressure (<1 Torr) and high temperature (>800° C.), or the like, areknown. Japanese Patent Unexamined Laid-open Publication No.S62(1987)-70295 discloses a method for manufacturing an n-typesemiconductor diamond thin film. According to the method, a reaction gascomprising a gas including P, As or Sb as a dopant element, ahydrocarbon gas and hydrogen is decomposed by heat or decomposed byplasma to be evaporated on a substrate by a microwave plasma CVD methodor a thermal decomposing CVD method, to thereby manufacture an n-typesemiconductor diamond thin film.

The above-mentioned plasma CVD method and thermal decomposing CVD methodhave drawbacks such that it is troublesome to execute the method due tothe high-temperature processing, the obtained film is easily broken dueto the residual stress maintained in the film, and it is difficult tocontrol an amount of dopant. As a method for solving these problems,Japanese Patent Unexamined Laid-open Publication No. H5(1993)-345696discloses a method for manufacturing a diamond thin film, which isexcellent in hardness, corrosion resistance, heat resistance, and thelike, and is possible to be applied to a high temperature electricequipment. In the method, a dopant ion beam is simultaneously introducedwhen a film is growing at low temperature by sputtering an ion beam.

Furthermore, a method for forming a p-type or n-type diamond is alsoknown (Japanese Patent Unexamined Laid-open Publication Nos.H5(1993)-117088 and H5(1993)-117089). In the method, an electron beam orexcimer laser is irradiated toward a surface of a single crystal diamondto activate a dopant disposed on the single crystal diamond, so that thedopant is diffused therein. Also known is an n-type semiconductordiamond in which nitrogen atom of 1×10¹⁹ cm⁻³ or more is doped (JapanesePatent Unexamined Laid-open Publication No. H7(1995)-69794).Furthermore, it is also known that, in forming an n-type diamond singlecrystal by a vapor-phase growth method, in order to prevent a reactionof gases due to corrosion thereof before reaching a substrate, each gasis directly supplied in a molecular flow state to the substrate(Japanese Patent Unexamined Laid-open Publication No. H10(1998)-149986).

Furthermore, the inventor invented a method for obtaining a singlecrystal diamond excellent in crystallization (Japanese Patent UnexaminedLaid-open Publication No. H9(1997) -20593). In the method, an amorphouscarbon hydride is formed by adding a hydrogen to a carbon, rapidlycooling a decomposed carbon gas hydride on a substrate, or sputtering agraphite by hydrogen atom, and atoms are rearranged into a crystaldiamond under low temperature by forming atomic holes and interstitialatom pairs in the amorphous carbon hydride, to thereby effectively causea movement of the interstitial atom. The single crystal diamond obtainedby this method meets characteristics required by various semiconductormaterials and optical semiconductor materials requiring a highlycontrolled crystallization.

(Problems to be Solved by the Invention)

A p-type diamond thin film having low resistance can be easilymanufactured by a conventional technique. Although an n-type diamondhaving high resistance can be manufactured, it is difficult tomanufacture an n-type diamond thin film having low resistance because itis impossible to activate at room temperature (300° K) due to theself-compensation effect and the deep donor level (500 meV).

If an n-type diamond having low resistance can be synthesized as asingle crystal diamond thin film, by combining it with a p-type diamondhaving low resistance, which is already realized by doping impurities,it is possible to manufacture a high power and high speed semiconductordevice using a diamond operable at high temperature and an ultravioletsemiconductor laser diode made of diamond, which is essential for a highdensity recording and a vast information transmittance.

It is also possible to manufacture a transparent n-type single crystalprotective film utilizing high hardness of a diamond, which is excellentin electric conductivity and thermal conductivity. Furthermore, byutilizing the negative electron affinity energy of a diamond, it ispossible to manufacture a display having a large surface area made byhighly efficient electron beam materials of an n-type diamond having lowresistance.

To form an n-type diamond having low resistance means to delete the deepdonor level due to N in a synthesized diamond and change the deep levelto a shallow level, whereby an absorption of natural light (sun light)is prevented, to thereby extinguish the color caused by the deep levelof the single N in the synthetic diamond. This is the same as in forminga transparent synthesized diamond by forming an n-type diamond havinglow resistance. In synthesizing a diamond by a high-temperature andhigh-pressure synthetic technique utilizing nickel catalyst or the like,it becomes possible to manufacture a transparent diamond valuable as ajewel.

(Means for Solving the Problems)

The inventor has found the facts that, in order to solve theaforementioned problems, in forming a single crystal diamond thin filmon a substrate by a vapor-phase growth method or a sputtering method, bysimultaneously doping a p-type dopant and an n-type dopant, it becomespossible to stabilize an n-type dopant in a high densitys, lower animpurity level and greatly increase the number of carriers, to therebysynthesize a high quality single crystal diamond thin film having lowresistance.

The inventor has also found the following fact. That is, in synthesizingan artificial diamond by a conventional high-temperature andhigh-pressure synthesizing method utilizing nickel catalyst or the like,by mixing H as a p-type dopant and N, P or As as an n-type dopant at theatomic density ratio of 1:2 to 1:3 before synthesizing, a donor-acceptorcompound, such as a P—H—P pair, an N—H—N pair or a As—H—As pair, isformed in a crystal, to lower the impurity level as compared to a singledoping to thereby form an n-type diamond having low resistance. Thisresults in a transparent artificial diamond, wherein a conventionalartificial diamond made by a conventional method had color because thenatural light is absorbed by the deep impurity level of N.

In other words, the method according to the present invention is basedon a principle that, regardless of a method for synthesizing a diamond,by simultaneously doping H as an acceptor and P, N, or As as a donor atthe atomic density ratio of 1:2 to 1:3, an acceptor-donor complex(compound) is formed, resulting in a decreased donor level. According tothis principle, a diamond in a metallic state (0.001 Ωcm) can also bemade.

As shown in FIG. 1, by forming a P—H—P pair, an N—H—N pair or a As—H—Aspair (complex) by a simultaneous doping, electron scattering due to ann-type carrier dopant is decreased, and the movement of the electron isgreatly increased. This lowers the donor level to thereby increase thecarrier density in a diamond crystal, which increases the activationrate by 10 to 1000 times, resulting in an n-type diamond having lowerresistance.

In the diamond crystal, H as an acceptor and N, P, or As as a donor takea structural position (an impurity complex) forming the crystal modelshown in FIG. 2. The positioning of the acceptor atom H between thedonor atom (P, N, As) and the donor atom (P, N, As) stabilizes thecrystallography structural position. Accordingly, donors can be doped inhigher density.

In the method according to the present invention, N, P or As as ann-type dopant in the form of atom and H as a p-type dopant in the formof atom electrically excited by a radio wave, a laser, an x-ray, anelectron beam, or the like, are simultaneously doped. Furthermore, acarbon vapor partial pressure, an n-type dopant partial pressure and ap-type dopant partial pressure are controlled so as to increase then-type dopant atomic density such that the atomic density of the n-typedopant is larger than that of the p-type dopant.

Furthermore, the present invention provides a method for removing H as adonor from the crystal. In the method, a single crystal thin film of thesynthesized diamond is cooled once, and annealed at high temperature fora short time in an electric field, so that the donor made of hydrogen isremoved from the crystal.

Furthermore, the present invention provides a method for forming a highefficiency spinpolarized electron-beam material. In the method, acircular polarized laser is irradiated to a single crystal thin film ofa synthesized diamond.

(Function)

By simultaneously doping an n-type dopant and a p-type dopant,electrostatic energy or lattice energy therebetween is decreased, whichstabilizes an n-type dopant and enables a stable doping of the n-dopantin high density, resulting in low resistance.

Furthermore, by simultaneously doping an n-type dopant and a p-typedopant, a pair of an n-type dopant and a p-type dopant is formed in adiamond crystal, which decreases electron scattering of n-type carrierdue to an n-type carrier dopant, to thereby increase the carriermovement, resulting in low resistance. In other words, according to thepresent invention, a diamond single crystal thin film having a filmthickness of about 0.01 to about 1 μm and a film resistance of 1.0 Ωcmor less can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing a principle that a donor levelbecomes lower by simultaneously doping a donor and an acceptor.

FIG. 2 is an explanatory view showing a donor-acceptor compound formedby simultaneously doping a donor and an acceptor.

FIG. 3 is a side view showing a concept of a device for executing asimultaneous doping method for forming a diamond thin film in accordancewith an MBE method.

THE BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a vapor-phase growth method denotes, forexample, an organometallic method utilizing a compound gas (MOCVDmethod) and a molecular beam epitaxy method utilizing an atomic beam(MBE method). A sputtering method denotes, for example, a sputtering ofa graphite by a hydrogen atom and other various methods applicable forsynthesizing a diamond thin film.

For example, in the MBE method, carbon gas hydride is decomposed undersufficient hydrogen conditions, and is accumulated to be crystalized ona semiconductor substrate at low temperature, so that an n-type singlecrystal diamond thin film of low resistance is formed. In place of themethod in which carbon gas hydride is decomposed, a method in whichhydrogen in the form of plasma is added to carbon may be used. In theMOCVD method or the MBE method, a gas which generates a large amount ofhydrogen such as tributylphosphine may be used as a doping material.

By simultaneously doping an atomic n-type dopant (donor) and an atomicp-type dopant (acceptor), a donor-acceptor pair is formed in a crystalto decrease the electrostatic energy or lattice energy therebetween,resulting in a stable doping of the n-type dopant in high density.Furthermore, since the donor-acceptor compound is formed, the donorlevel greatly decreases. Thus, high power and high speed diamondsemiconductor device materials operable at high temperature, or highefficiency electron beam materials, can be manufactured.

FIG. 3 shows an example of a schematic side view of a device forexecuting an MBE method as one of the methods according to the presentinvention.

A diamond single crystal substrate 2 is fixed to a holder (not shown).The substrate 2 is heated to 300° C. to 950° C. by an electric heater(not shown) in a state that the vacuum chamber 1 is maintained in avacuumed state by a vacuum evacuating device (not shown). Carbon hydridegas or hydrogen added carbon is introduced through a gas introducingpipe 4, and thermally decomposed by a RF coil 7 to be supplied in amolecular flow toward the substrate 2 so as to be adsorbed on thesubstrate 2.

A donor such as N, P or As is thermally decomposed by the RF coil 7, andis supplied toward the substrate 2 through the gas introducing pipe 5.Simultaneously, an acceptor such as H is doped by supplying it throughthe gas introducing pipe 6 toward the substrate 2, to thereby form adiamond 3 on the substrate. N, P or As as an n-type donor and H as ap-type acceptor may be atomic gas formed by irradiating anelectromagnetic wave in a micro wave region to a molecular gas, or anatomic simplicial cell formed by heating at high temperature.

In forming the crystal, a carbon vapor partial pressure, an n-typedopant partial pressure and a p-type dopant partial pressure arecontrolled so as to increase the n-type dopant atomic density such thatthe atomic density of the n-type dopant is larger than that of thep-type dopant. In other words, the ratio of the atomic density of thep-type dopant to the n-type dopant is controlled such that the atomicdensity of the n-type dopant is larger than that of the p-type dopant,such as 1:2 to 1:3, to thereby form an n-type impurity compound in whichthe atomic density of the p-type dopant to the n-type dopant is 1:2 to1:3.

Furthermore, after forming an n-type single crystal diamond thin filmhaving low resistance on the semiconductor diamond substrate 2 under lowtemperature and low pressure, the thin film is once cooled, and annealedat high temperature for a short time in an electric field, so that theacceptor made of hydrogen is removed from the crystal so as to restore apassivation by a hydrogen. At this time, the amount of hydrogen acceptoras a p-dopant which easily moves in the crystal is controlled byelectron exciting (electron beam irradiation) and thermal exciting suchthat the atomic density of the n-type dopant is a little bit larger thanthat of the p-type dopant, such as the atomic density ratio of the donorto the hydrogen acceptor is from 2:1 to 3:1.

Furthermore, an n-type single crystal diamond thin film having lowresistance is formed by simultaneously doping a p-type dopant and ann-type dopant, to realize a negative electron affinity energy. Thiscauses a higher level of electrons than a vacuum level. As a result, thediamond thin film can be used as an electron emitter for a large screendisplay, or the like. By irradiating a circular polarized electron beamto the n-type single crystal diamond thin film having low resistanceshowing the negative electron affinity energy to differentiate thenumber of electrons between the upwardly oriented spin and thedownwardly oriented spin, electrons having a differentiated orientedspin can be taken out at low voltage. Thus, a highly efficientspin-polarized electron beam material can be manufactured.

In synthesizing an artificial diamond by a conventional high-temperatureand high-pressure synthesizing method utilizing a nickel catalyst, an Nor P compound including a large amount of H such as NH₃ in a non-diamondcarbon such as a graphite, or an As compound is disposed in ahigh-temperature and high-pressure device. Then, H as a p-type dopantand one of N, P and As as an n-type dopant are mixed so that the atomicdensity ratio of the p-type dopant to the n-type dopant becomes 1:2 to1:3 under normal high-temperature and high-pressure manufacturingconditions.

EXAMPLES

As shown in FIG. 3, the vacuum chamber 1 was maintained to be vacuumdegree of 10⁻¹⁰ torr, and the substrate 2 was heated by the electricalheater. CH gas (hydrocarbon, methane, ethane, propane) was introducedthrough the gas introducing pipe 4, and was thermally decomposed by theRF coil 7 to supply the molecular flow toward the substrate 2 so as tobe adsorbed on the substrate 2. P as a donor was emitted toward thesubstrate 2 through the introducing pipe 5 at the flow rate of 5×10⁻⁹torr, and H as an acceptor was emitted toward the substrate 2 from theintroducing pipe 6 at the flow rate of 10⁻⁹ torr so as to simultaneouslydope them, to thereby form a diamond 3 at the substrate temperature of400° C., 450° C., 600° C., 800° C. and 900° C. P as a donor and H as anacceptor were electrically excited by the RF coil 7 so as to become anatomic gaseous state. After 90 minutes had passed, the crystal growthwas interrupted.

The obtained single crystal diamond thin film had thickness as shown inTable 1. In a case of simultaneous doping of P and H, the n-type carrierdensity was higher by a few digits at any crystal growth temperatures,as compared to the case where only the P vapor as a n-type donor wasdoped without feeding H vapor as a p-type dopant.

Furthermore, the donor density was different depending on the crystalgrowth temperature (the substrate temperature). The donor density was1×10²⁰ cm⁻³ or more at the substrate temperature of 800° C. In addition,the film resistance was 1.0 Ωcm or less, as shown in Table 1.

TABLE 1 Sub- strate Film Simultaneous doping of P and H Single doping ofP Tem- thick- Film Donor Film perature ness Donor resistance densityresistance (° C.) (μm) density (cm⁻³) (Ω cm) (cm⁻³) (Ω cm) 400 0.01 4 ×10¹⁸ 0.83 1 × 10¹⁴ 4120 450 0.09 5 × 10¹⁸ 0.52 2 × 10¹⁴ 3150 600 0.21 6× 10¹⁹ 0.45 3 × 10¹⁴ 2180 800 0.82 8 × 10²⁰ 0.35 3 × 10¹⁴ 1210 900 1.038 × 10²¹ 0.20 4 × 10¹⁵  400

Industrial Applicability

According to the method of the present invention, an n-type diamond canbe synthesized as a single crystal diamond thin film. Therefore, it ispossible to manufacture a high power and high speed semiconductor deviceusing a diamond operable at high temperature, and an ultravioletsemiconductor laser diode essential for a high density recording and avast information transmittance by a diamond.

It is also possible to manufacture an n-type transparent single crystalprotective film having low resistance, which utilizes high hardnesscharacteristics of a diamond. Furthermore, since an n-type singlecrystal diamond thin film having low resistance can be manufactured bysimultaneously doping a p-type dopant and an n-type dopant, negativeelectron affinity energy can be obtained. Thus, by irradiating acircular polarized laser to the thin film, it is possible to manufacturea high-power and high-speed display operable at high temperature andhaving a large screen area made by highly efficient electron beammaterials.

What is claimed is:
 1. A method for synthesizing an n-type diamondsingle crystal thin film having low resistance, the method including thesteps of: forming a single crystal diamond thin film on a substrate by avapor-phase growth method or a sputtering method; and simultaneouslydoping N, P or As as an n-type dopant, and H as a p-type dopant in acrystal to form a donor-acceptor pair in the crystal, to therebysynthesize the n-type diamond single crystal thin film having lowresistance.
 2. The method for synthesizing an n-type diamond singlecrystal thin film having low resistance as recited in claim 1, whereinan atomic N, P or As as an n-type dopant and an atomic H as a p-typedopant electrically excited by a radio wave, a laser, an x-ray, or anelectron beam, are simultaneously doped.
 3. The method for synthesizingan n-type diamond single crystal thin film having low resistance asrecited in claim 1, wherein carbon vapor partial pressure, n-type dopantpartial pressure and p-type dopant partial pressure are controlled suchthat the n-type dopant atomic density becomes larger than the p-typedopant atomic density.
 4. A method for restoring a passivation by ahydrogen, including the steps of: subjecting a single crystal diamondthin film synthesized by the method as recited in claim 1 to cooling;and then annealing it at high temperature for a short time in anelectric field, so that the donor made of hydrogen is removed from thecrystal.
 5. A method for forming a high efficiency spin-polarizedelectron-beam, including the step of: irradiating a circular polarizedlaser on the single crystal diamond thin film synthesized by the methodrecited in claim
 1. 6. A method for synthesizing an n-type diamondhaving low resistance, including the step of: mixing H as a p-typedopant and N, P or As as an n-type dopant so that the atomic densityratio of the p-type dopant to the n-type dopant is 1:2 to 1:3 beforesynthesizing the materials, in synthesizing an artificial diamond by ahigh-temperature and high-pressure synthesizing method utilizing acatalyst, to thereby form a donor-acceptor pair in the crystal.
 7. Themethod for synthesizing an n-type diamond single crystal thin filmhaving a low resistance as recited in claim 1, wherein N is the n-typedopant.
 8. The method for synthesizing an n-type diamond single crystalthin film having a low resistance as recited in claim 1, wherein P isthe n-type dopant.
 9. The method for synthesizing an n-type diamondsingle crystal thin film having a low resistance as recited in claim 1,wherein As is the n-type dopant.
 10. The method for synthesizing ann-type diamond single crystal thin film having a low resistance asrecited in claim 1, wherein the donor intensity is 1×10²⁰ cm⁻or more atthe substrate temperature of 800° C.
 11. The method for synthesizing ann-type diamond single crystal thin film having a low resistance asrecited in claim 1, wherein the film resistance is 1.0 Ωcm or less.